1. Technical Field
The present disclosure generally relates to well bore tools and in particular to apparatus and methods for downhole fluid sampling.
2. Background Information
In the oil and gas industry, formation testing tools have been used for monitoring formation pressures along a well borehole, obtaining formation fluid samples from the borehole and predicting performance of reservoirs around the borehole. Such formation testing tools typically contain an elongated body having an elastomeric packer and/or pad that is sealingly urged against a zone of interest in the borehole to collect formation fluid samples in fluid receiving chambers placed in the tool.
Downhole multi-tester instruments have been developed with extensible sampling probes for engaging the borehole wall at the formation of interest for withdrawing fluid samples from the formation and for measuring pressure. In downhole instruments of this nature an internal pump or piston may be used after engaging the borehole wall to reduce pressure at the instrument formation interface causing fluid to flow from the formation into the instrument.
Formation fluid sampling during drilling operations requires a clean-up process for removing contaminants, such as borehole fluid containing drilling fluid and formation fluid, from the fluid entering a sampling tool. Likewise, sampling processes using wireline tools need to remove contaminants from fluid samples. Once the clean-up process is complete, a sampling process may begin where clean formation fluid is transferred to sampling chambers. Traditionally, an application engineer at the well site sends a command to switch over from the clean-up process to the sampling process. The clean-up process is sometimes a timed process whereby the application engineer waits a predetermined length of time and assumes that the fluid stream entering the tool is free of contaminants at the end of the time period for clean-up. Some sampling processes include fluid measurements that are interpreted by an application engineer who then decides when to switch from the clean-up process to the sampling process. Pump rates are typically selected as a fixed rate low enough to maintain pressure above the fluid bubble-point during the draw down.